The cutting of materials with the aid of cutting electrodes energized by a suitable power source is a known technique that is being successfully employed, e.g., in the field of electrosurgery. Typical electrosurgical devices apply a electrical potential difference or a voltage difference between a cutting electrode and a patient's grounded body (monopolar arrangement) or between a cutting electrode and a return electrode (bipolar arrangement) to deliver electrical energy to the area where tissue is to be cut. The voltage is applied either as a continuous train of high frequency pulses, typically in the RF range, or as direct current (DC).
The prior art provides a number of exemplary designs of bipolar electrosurgical electrodes. For example, U.S. Pat. No. 5,108,391 describes a bipolar treating apparatus with a first active electrode and a second return electrode having exposed distal ends to define a bipolar tip for electrosurgically treating tissue. U.S. Pat. No. 5,700,262 describes a bipolar electrode with fluid channels for performing neurosurgery. Additional information about bipolar electrosurgical devices and knives can be found, e.g., in U.S. Pat. Nos. 4,202,337 and 4,228,800 as well as numerous other open literature sources.
Depending on the conditions, the application of a voltage to a monopolar electrode or between the cutting and return electrodes of a bipolar electrode produces a number of physical phenomena. Most prior art devices take advantage of one of these phenomena to perform the cut. In particular, one class of devices uses a gas stream that is generated around the cutting electrode. For example, U.S. Pat. No. 5,217,457 describes an electrosurgical apparatus using a stream of gas that shrouds the electrode and an electrosurgical apparatus incorporating this electrode for cutting biological tissue. U.S. Pat. No. 5,088,997 also teaches the use of a stream of gas for electrosurgical procedures for coagulating or cutting biological tissue. On the other hand, U.S. Pat. No. 5,300,068 teaches an electrosurgical apparatus for cutting tissue and for ablating occlusions using arc discharges produced on a monopolar electrode in response to a train of pulses. Taking advantage of a yet different phenomenon, U.S. Pat. No. 6,352,535 teaches a method and device for electro microsurgery in a physiological liquid environment that uses high voltage electrical discharges of sub-microsecond duration in a liquid medium to produce cavitation bubbles. The cavitation bubbles have a size in the sub-millimeter range and are used for high-speed precision cutting with an inlaid disc electrode.
In addition to taking advantage of different phenomena to perform the cut, prior art devices employ various techniques for generating and applying the voltage to the electrode or electrodes. U.S. Pat. No. 6,135,998 teaches an electrosurgical device which uses extremely short monopolar voltage pulses, typically shorter than 200 ns, to drive an electrode having an inlaid disc geometry. This invention attempts to mitigate some of the negative cavitation effects, such as the damaging jets formed after the collapse of the cavitation bubble. U.S. Pat. No. 5,108,391 describes a high frequency generator for tissue cutting and for coagulating in high-frequency surgery. This device uses an electric arc discharge to perform the cutting operation. U.S. Pat. No. 6,267,757 teaches a device which uses radio-frequency (RF) ablation for revascularization. It employs a source, which delivers at least one burst of RF energy over an interval of about 1 to about 500 ms, and preferably about 30 to about 130 ms. This device has an elongated insulated, electrical conducting shaft with an uninsulated distal tip, which is configured to emit the RF energy. U.S. Pat. No. 6,364,877 also describes the use of high frequency pulses applied in a continuous manner. The teaching found in U.S. Pat. Nos. 5,697,090 and 5,766,153 suggests that a continuous train of high frequency pulses can be pulsed at a rate sufficient to allow the electrode to cool.
Unfortunately, despite all the above teachings, electrosurgical methods and apparatus generally suffer from an inability to control the depth of tissue damage (necrosis) in the tissue being treated. Most electrosurgical devices described above rely on a gas jet, an arc discharge or cavitation bubbles to cut, coagulate or ablate tissue. Such imprecise cutting methods cause tissue necrosis extending up to 1,700 μm into surrounding tissue in some cases.
In an effort to overcome at least some of the limitations of electrosurgery, laser apparatus have been developed for use in arthroscopic and other procedures. Lasers do not suffer from electrical shorting in conductive environments and certain types of lasers allow for very controlled cutting with limited depth of necrosis. U.S. Pat. No. 5,785,704 provides an example of a laser used for performing stereotactic laser surgery. Unfortunately, lasers suffer from limitations such as slow operating speed, inability to work in liquid environments, high cost, inconvenient delivery systems and other defects that prevent their more universal application. For these reasons, it would be desirable to provide improved apparatus and efficient methods for driving an electrosurgical apparatus for ablating tissue in a highly controlled and efficient manner while minimizing tissue damage.
The prior art has attempted to provide for more controlled electrosurgery by relying on plasma-mediated cutting and ablation of soft biological tissue in conductive liquid media at low temperatures. The fundamentals of this approach, which is used predominantly in the continuous pulse regime, and various embodiments employing it are described in the patents of Arthrocare including U.S. Pat. Nos. 5,683,366; 5,697,281; 5,843,019; 5,873,855; 6,032,674; 6,102,046; 6,149,620; 6,228,082; 6,254,600 and 6,355,032. The mechanism of low temperature ablation is called “coblation” and is described as electric field-induced molecular breakdown of target tissue through molecular dissociation. In other words, the tissue structure is volumetrically removed through molecular disintegration of complex organic molecules into non-viable atoms and molecules, such as hydrogen, oxides of carbon, hydrocarbons and nitrogen compounds. This molecular disintegration completely removes the tissue structure, as opposed to transforming the tissue material from solid form directly to a gas form, as is typically the case with ablation (see U.S. Pat. No. 5,683,366). More specifically, this mechanism of ablation is described as being associated with two factors: (1) “photoablation” by UV light at 306-315 nm and visible light at 588-590 nm produced by the plasma discharge; and (2) energetic electrons (e.g. 4 to 5 eV) can subsequently bombard a molecule and break its bonds, dissociating a molecule into free radicals, which then combine into final gaseous or liquid species (see U.S. Pat. No. 5,683,366). Surface temperature of tissue in this process is maintained between 40-70° C. This type of ablation mechanism has low rate of tissue dissection and a very limited applicability to hard tissues such as, for example, bones.
Despite these new advances the electrosurgical techniques are still experiencing a number of problems remain. First and foremost, the amount of power required to operate the prior art cutting devices remains in a high range of several Watts which precludes applications of these devices to such delicate organs as an eye. Second, the devices exhibit large energy and heat losses. These high losses translated into excessive power deposition into the tissue being ablated. Additional heat losses to the hand piece are also substantial. Third, even the best prior art devices operating at the lowest power levels have difficulties cutting hard biomaterials like bones and non-conducting materials such as cellulose or plastics.
Increasingly sophisticated surgical procedures create a growing demand for more precise and less traumatic surgical devices. The critical importance and delicate nature of the eye makes the demand for precision and safety of intraocular microsurgical instrumentation particularly important. For these and other reasons, it would be a major advance in the art to provide an apparatus and method for ablating materials at low power levels. It would be particularly useful to provide such apparatus and method that reduces heat losses to the material being cut as well as into the surroundings and, especially the hand piece. Furthermore, it would also be an advance to expand the range of materials that can be ablated to include biological tissue, cellulose and plastics.